The chain saw includes an engine or an electric motor as a drive source. An engine chain saw is generally provided with a centrifugal clutch. When the rotation number of a drive shaft exceeds a setting value, the centrifugal clutch is turned on, so that power is transmitted from the drive shaft to the chain blade and thus the chain blade is rotationally driven. Further, when the rotation number of the drive shaft is less than the setting value, the centrifugal clutch is turned off, so that power transmission from the drive shaft to the chain blade is shut off and thus the chain blade remains in a stopped state.
When rebound of the chain saw occurs during cutting an object to be cut such as the wood, a hand guard is collapsed forward by a hand grasping a handle and thus a chain brake is activated, so that the rotation of the chain blade along a guide bar can be stopped. Incidentally, the chain saw using an electric motor as a drive source includes a mechanical brake which is actuated by an operation of the hand guard or an electrical brake in which driving of the electric motor is stopped by releasing a trigger switch, as disclosed in JP-A-55-18031.
However, when a brake mechanism is actuated by pivoting the hand guard, it is necessary for an operator to move the hand guard and simultaneously return a trigger in order to brake, even if the brake device is installed. Such an operation is cumbersome and thus there is a demand to automatically actuate the brake.